BookScouter.com "helps you to sell your books for the highest price. It compares prices from over 40 book-buying websites so that you can quickly find the site that is paying the most for your books."
They show you a list of websites and how much you will receive from each with a link to the pages. It's convenient, free, and there's NO REGISTRATION.
It's as simple as open the webpage. Type the ISBN. Click search and you're done!! Then, you click the website of the highest price offered and it brings you to that site. Brilliant!
*I am in no way affiliated, paid, or benefit from this post. This is simply my opinion and not to be taken as anything else.*